In recent years, rechargeable electric storage devices such as battery cells (such as lithium ion battery cells and nickel hydrogen battery cells) and capacitors (such as electric double layer capacitors) are employed as power sources of vehicles (such as automobiles and motorcycles) and various devices (such as mobile terminals and notebook computers). For example, there are various types of battery cells. As one of the types, there is provided a battery cell including an electrode assembly (power generating element), a current collector electrically connected to the electrode assembly, a case (battery case) housing the electrode assembly and the current collector, an external terminal (terminal plate) arranged outside the case, an internal sealing member (gasket) arranged along an inner surface of a partition wall of the case, an external sealing member (gasket) arranged along the outer surface of the partition wall of the case, and a rivet member (terminal) inserted through the partition wall of the case, the internal sealing member, and the external sealing member, wherein the external terminal and the current collector are electrically connected to each other via the rivet member.
The rivet member is composed of a conductive material. The rivet member includes a body part (body) in the form of a quadrangular prism, a first insert part (upper shaft) in the form of a solid shaft provided continuously to the body part, and a second insert part (lower shaft) in the form of a hollow shaft (cylindrical shape) provided continuously to the body part. The first insert part and the second insert part each have an outer diameter smaller than the outer diameter of the body part. While the first insert part is inserted through the external terminal, the distal end of the first insert part is swaged. The distal end of the second insert part is swaged while the second insert part is inserted through the partition wall of the case, the internal sealing member, the outer sealing member, and the current collector (see Patent Literature 1, for example).
Thus, the distal end of the first insert part and the distal end of the second insert part of the rivet member each have an expanded diameter. That is, the first insert part in the form of a solid shaft is deformed to have a larger diameter due to its distal end being entirely crushed. On the other hand, the second insert part in the form of a hollow shaft is deformed to have a flange shape due to its distal end being crushed while inclined outwardly in the radial direction. Therefore, the swaged distal ends of the rivet member deform and expand on the conductive members such as the external terminal and the current collector, resulting in an increase in thickness of the external terminal and the current collector. From the viewpoint of the energy density per volume of the electric storage device, it is better for the conductive members to have a smaller apparent thickness.
Further, this problem applies not only to battery cells but also to capacitors (such as electric double layer capacitors).